Partially-purified gentamicin adenyl transferase (GAT) has been employed in an assay for gentamicin in human body fluids. This assay is more rapid and accurate than microbiologic methods. GAT can be further purified by affinity chromatography. Attempts to utilize an R factor beta-lactamase for clinical assay have revealed technical problems in the sensitivity of the systems studied. The properties and potential utility in clinical assays of an aminoglycoside are currently being studied.